This invention relates to a method for making thin film electroluminescent devices and, more particularly, to a sputtering method for producing electroluminescent films and devices which are operable at low dc voltages.
Electroluminescent devices which can be operated at low dc voltages are useful because of their compatibility with integrated circuit devices. Furthermore, such devices can be operated from batteries without the hazard associated with higher operating voltages. Various phosphor materials have been used in dc electroluminescent devices. Also, various methods have been utilized to fabricate dc electroluminescent devices. Thornton, Journal of Applied Physics, Volume 33, pp. 3045-3048, 1962, and later Plumb, Japanese Journal of Applied Physics, Volume 10, pp. 326-331, March 1971, evaporated ZnS:Mn, Cu, Cl phosphor materials, on a glass substrate coated with tin oxide or indiumtin oxide. Abdalla and Thomas, Proceedings of the S.I.D., Volume 19/3, pp. 91-95, 1978, prepared films of ZnS:Mn, Cu by evaporating Zn, S, Mn, and Cu onto a tin oxide or indium-tin oxide coated glass substrate from separate sources.
Sputtering of electroluminescent cells was described by Hanak in Proc. 6th Internl. Vacuum Congr., 1974, Japanese Journal of Applied Physics Suppl. 2, Pt. 1, pp. 809-812, 1974. One disclosed method of making electroluminescent films consists of sputtering from a composite target of zinc sulfide having numerous squares of manganese and copper placed over it in a regular pattern. The resulting electroluminescent film composition is determined by the relative areas of the manganese, copper, and zinc sulfide in the composite target. Another disclosed method, found to be undesirable, utilizes a premixed powder target of the desired composition.
One disadvantage of fabricating electroluminescent devices by evaporation or by the above-described sputtering techniques is the difficulty in maintaining close control over the stoichiometry and doping levels of the electroluminescent film. The composition of the electroluminescent film is critical in establishing its operating parameters. A further disadvantage of the above-described methods is the inability to produce a graded dopant profile within the electroluminescent film.